The prior art includes a very large number of patents dealing with both mechanical and hydraulic pumping units, and a considerable number of the mechanical types employ some sort of mechanism to utilize the kinetic energy from deceleration and stopping of each stroke for beginning and accelerating the next stroke. To date the prior art has not shown us a satisfactory hydraulic or hydraulic-pneumatic method for this transfer of energy from one stroke to the next.
The walking beam pumping unit, by far the most popular and successful unit for many years, performs a very smooth reversal and is efficient at transferring the kinetic energy from each stroke to the next. Its motor functions constantly, which is advantageous, but there are power peaks due to the rotary drive connection to the beam, which is a disadvantage, and requires a larger motor.
The problem of parasitic rod string oscillation is pronounced in the walking beam unit, and its stroke velocity must be restricted to minimize the effects, and at the same time limiting production.
The large gearbox required to drive the reciprocating beam is a major drawback, and the mechanics of the system seem to generally limit its stroke length to less than twenty feet. For these reasons there have been many attempts by inventors and others associated with the production industry to produce a pumping unit in which power is applied more linearly, eliminating the beam and large gearbox, and in which a longer stroke and higher stroke velocity are easily accommodated.
These prior designs are mechanical, hydraulic, or a combination of the two, and are varied. Many have some good features, but all have shortcomings.
Most obvious among these shortcomings is the absence of an efficient stroke reversal mechanism in combination with a linear drive mechanism which provides a constant velocity for the remainder of the stroke cycle, eliminating power peaks and allowing a small, and therefore efficient, motor.
None of the designs within the field of the present invention incorporates features which eliminate parasitic rod string oscillation due to stroke reversal. All of these prior pumping units cause the reversal of the polish rod and upper end of the rod string with little regard for the current forces upon the lower end of the rod string due to rod stretch and inertia, which results in the upper end of the rod string reversing from upward to downward movement while the lower end of the rod string is still moving upward, and, respectively, reversing from downward movement to upward movement at its upper end while its lower end is still moving downward.
Many of the prior art pumping units, particularly the totally hydraulic ones, have a large number of control components, and many have special components which are difficult and expensive to manufacture.
Most prior hydraulic units have no stroke reversal capability, and they waste the kinetic energy of each stroke, thus requiting a larger motor to begin the next stroke from a dead stop.
Additionally, many pumping units of the prior art use solid state control devices, which presents a problem because of the possibility of damage to them due to power surges or electrical storms.